Structure of the Buller and Takaka Terrane rocks adjacent to the Anatoki Fault, northwest Nelson, New Zealand

Abstract

Structures and microstructures within flanking lithologies of the Buller and Takaka Terrane boundary, the Anatoki Fault, record three pre‐Cenozoic deformation events in northwest Nelson. Each deformation event recognised in the Buller Terrane has structures that can be matched in style, orientation, and timing of development with that in the adjacent Takaka Terrane. D 1 is represented by north‐trending, upright or overturned to the west, large‐scale folds with an axial planar slaty cleavage. D 1 is mid Devonian in age and relates to amalgamation of the Buller and Takaka Terrane. D 2foliation occurs in a zone of ductile deformation adjacent to the Anatoki Fault near Boulder Lake. Rb‐Sr geochronology, and the relationship between the foliation and metamorphism associated with the adjacent c. 111 Ma Mt Olympus Pluton, suggests D 2 formed in the Early Cretaceous following pluton emplacement. D 3 is represented mainly by mesoscopic folds with an axial planar crenulation cleavage. D 3 structures crenulate and refold both D 1 and D 2 structures and are mid Cretaceous in age. Both D 2 and D 3 relate to an extremely active and changing tectonic period of New Zealand in the Early to mid Cretaceous. The east‐dipping Anatoki Fault in northwest Nelson records a complex history of ductile and brittle movement. Tectonites from central segments record ductile/brittle east‐over‐west reverse‐slip associated with D 1. To the north, D 2 tectonites from the Boulder Lake area record Early Cretaceous ductile dextral‐slip reactivation. To the south, tectonites from the Crow River and Mt Benson area record ductile/brittle dextral‐normal slip which, in the Crow River area, represents reactivation that postdates the intrusion of the c. 137 Ma Crow Granite. From the Crow River southwards, the Anatoki Fault has also undergone late Cenozoic brittle reactivation.